1. Field of the Invention
The present invention is directed to a rotating anode X-ray tube suitable for use as a high-power X-ray tube for use in CT (computed tomography) installations.
2. Description of the Prior Art
It is problematical in rotating anode X-ray tubes that the focal path of the target, i.e. the anode surface onto which the electron beams emitted by the cathode are incident, is subject to constant wear during tube operation. This wear leads to a modification of the spectral composition of the X-rays, which usually are emitted in a flat angle. Also, the usable X-ray dose is diminished because of the radiation emitted at the flat angle. Another problem arises due to the stripping of particles from the focal path because increased arcing can occur in the X-ray tube.
In order to reduce the wear of the focal path, it is known to add rhenium to the anode material, usually tungsten. This measure in fact has a beneficial effect on the durability of the anode but does not eliminate the other problems that have been mentioned.
German Patent 890 246 discloses a rigidly arranged anode wherein the focal path of the anode be fashioned as a circulating metallic liquid. Such a fashioning is supposed to have the advantage that the focal spot is constantly replenished and that the X-ray tube can be placed under a substantially greater load. Mercury is cited as liquid in the this reference, this being arranged in an evacuated, closed vessel of, for example, glass. During operation of the tube, the vessel is placed into rotation by means of a rotating electromagnetic field, and the metallic liquid forms a paraboloid of revolution upon rotation under the influence of the rotating field. An opening through which the electrons coming from the cathode can proceed onto the outside surface of the mercury body is situated in the wall of the vessel that accepts the metallic liquid. The X-ray beam generated in this way has the approximate shape of a cone. A collecting device is provided for collecting the mercury that splatters through this opening during rotation, this collecting device also having a space for the condensation of the mercury vapor.
It is obvious that such a fashioning and arrangement of a liquid anode is not only very complicated in design terms but also is already very problematical in and of itself because of the mercury, and is technically not suited for high-power tubes because of the high vapor pressure of the mercury.
Other embodiments of X-ray tubes with a liquid metallic target that have been more recently disclosed (U.S. Pat. Nos. 6,185,277; and 5,052,034) also are affected by the aforementioned disadvantages; and also are not suited for utilization in rotating anode X-ray tubes of the type initially described.
An object of the present invention is to provide a rotating anode X-ray tube that, in particular, can be used as a high-power tube in a CT installation, with which the aforementioned disadvantages are avoided and with which, in particular, the service life of the anode can be lengthened in an economical way.
The above object is achieved in accordance with the present invention in a rotating anode X-ray tube, particularly of the type suitable for use in CT installations, having a housing with a cathode rigidly arranged therein and an anode rotatably arranged therein, and wherein the anode has a focal path composed of target material that melts during operation of the tube, and wherein, when the target material is in a molten state, it is held to the anode by a rotary body due to centrifugal forces of the rotary body when rotating around the cathode.
The invention is based on the perception of melting the focal path of the anode during operation and thus constantly smoothing it, i.e. keeping it smooth during operation. To avoid the molten material from being thrown off from the anode due to centrifugal forces given rotation of the anode, the rotary body is designed such that the centrifugal forces hold the molten material in place and simultaneously prevent the formation of a parabolic surface.
The inventive anode X-ray tube is comparatively simple to fashion in terms of design and requires no collecting vessel as in the aforementioned known tubes.
In an advantageous embodiment, the anode is arranged at a rotary body that has an annular focal path. The rotary body can be cylindrical or funnel-like, and the anode with the focal path is arranged at the expanded end of the rotary body given the latter design.
Tungsten can be provided as the target material to be melted. Since, however, tungsten already has a vapor pressure above its melting point that could lead to arcing and emission problems given a standard design of a cathode/anode arrangement, a eutectic tungsten alloy having a melting point below that of tungsten and having a vapor pressure of  less than 0.1 hPa in the environment of the melting temperature is advantageously employed.
The following elements are suitable as further materials for the focal path to be melted during operation: tantalum (Ta), osmium (Os), ruthenium (Ru), molybdenum (Mo), niobium (Nb), rhodium (Rh), thorium (Th), palladium (Pd), gold (Au), iridium (Ir), rhenium (Re), platinum (Pt), hafnium (Hf), lanthanum (La). An alloy system of subsets of these elements or boride or carbide compounds of said elements can also be advantageously applied.
In a further version of the invention, the cathode and anode are shielded from one another by partitions and that the cathode thus is protected against vapors from the anode material. The partitions are advantageously provided with a diaphragm in the region of the beam passage that blocks the vapor pressure but is largely transmissive for electrons having kinetic energies that typically lie above approximately 60 keV in this application. Such a version has the advantage that even pure tungsten can be employed as the meltable target material; in particular, materials having higher vapor pressures, and thus materials having higher melting temperatures than, for example, tungsten, can be permitted.